One of the largest study of prenatal smoking found that babies born to mothers who smoke while pregnant increase substantially the chance of early neurological development and the effect may be stronger than previous research studies.

Researchers found that smoking may cause as much as 40 percent increase in chance that infant may risk having developmental problems between 3 and 24 months old. The effects were strongest among children from poor families, published in the Journal of Human Capital.

"This study underscores the dangers of prenatal smoking," said George Wehby, a professor at the University of Iowa's College of Public Health and the study's lead author. "We hope it also highlights the need for continued efforts to discourage expectant mothers from smoking."

Study participants were recruited from health clinics in South America including Brazil, Chile, Argentina. Nearly 1,600 children were included, making it one of the largest studies of prenatal smoking and neuro-development.

Researchers surveyed mothers about their smoking habits, and physicians conducted neurological screening; including cognitive test, assessment of communication and basic neurological function on their children. Close to 11 percent of mothers in the sample had smoked during their pregnancy.

Previous studies have found similar adverse effects of prenatal smoking, but no previous work has tried to isolate the smoking effect from other factors such as drinking and other activities that can harm their babies.

Researchers used statistical technique to help account for biases, which are difficult to observe directly. They took advantage of geographical smoking behaviors of mothers, variations of smoking policies and cigarette prices. The control allowed researchers to directly pin-point the effects of smoking.

Although there is an increased awareness of the dangers of smoking, rates of prenatal smoking remain high. According to the Center for Disease Control and Prevention in 2005, 12 percent of pregnant American women reported that they smoked while pregnant.

"Given the importance of early child health and neurodevelopment for future wellbeing, targeted interventions to reduce prenatal smoking may result in significant improvements in child development and long-run human capital, “said Wehby.